As is known, rail-cars (powered railway carriage, coaches, etc.) of surface and underground trains have, at their ends, collapsible devices pre-arranged for absorbing energy in the event of head-on collision so as to safeguard the body that accommodates the passengers and/or drivers. In general, a collapsible device comprises a box-like body made of metal material coupled to an end plate, usually provided with anti-climbing ribbings. In the event of a head-on collision against said end plate, the box-like body undergoes plastic deformation and hence absorbs kinetic energy.
Normally, during a head-on collision, the collapsible devices are not perfectly aligned with the ones against which they impact, but an offset in a vertical direction or else an angular offset is present between their axes. These offsets generate an asymmetrical distribution of the load between the collapsible devices, so that the conditions of impact and the amount of energy absorbed differ from what would be expected according to design.
In order to overcome these drawbacks, it is known to provide a guide device inside the box-like body of the collapsible device. For example, the patent EP2011713 illustrates a guide device having a series of vertical diaphragms, which are set at a distance apart, are perforated axially and are engaged by a stem fixed to the end plate. During impact, the stem recedes and slides into the diaphragms, which thus guide it so as to limit any rotations of the end plate.
The known solutions just described a not satisfactory in so far as: they frequently require a free space behind the box-like body for housing the stem at the end of plastic deformation; they comprise a relatively high number of components on account of the guide device; and they have a relatively high weight.
In order to reduce the weight of the box-like bodies, it is known to use composite materials, instead of metal materials, but in these cases there arises the problem of managing to obtain the same performance levels provided by traditional collapsible devices made of metal material.
As regards collapsible devices made of composite material, DE19526119A describes a solution that corresponds to the preamble of Claim 1 and that comprises a first tube, which is supported by the frame of a vehicle and defines a guide for a second tube.
The rear end of the second tube is housed in the first tube and is axially aligned with a serrated element, which crushes, or shatters, the second tube when the latter recedes as a result of a head-on collision. Progressive crushing, or shattering, of the composite material can be likened to plastic deformation of metal materials, in so far as both of these phenomena of collapse enable energy absorption.
In DE19526119A, the front portion of the second tube projects with respect to the first tube and supports another serrated element, which starts to crush the first tube when it reaches the free end of the latter. Starting from this instant, also the first tube collapses, crushing starting from its free end. In particular, the profile of both of the tubes may have a cross section that varies along their axial dimension to obtain a desired gradient of energy absorbed during impact.
Also this solution, however, presents some drawbacks.
In the first place, during a first step of the impact only the second tube is crushed and absorbs energy so that energy absorption is not maximized. In addition, at the point where also the first tube must start to be crushed, the characteristic curve that defines the compressive strength of the collapsible device presents a sharp variation, which may entail an anomaly in the real behaviour of the collapsible device as compared to the behaviour expected according to design.
The second tube is relatively long and slender and, on account of its front cantilever portion, is not adequate to withstand a direct impact with an offset with respect to its axis. In fact, with a misaligned or inclined load, given that said front portion is not directly constrained to the first tube, it undergoes bending, which could cause failure of the second tube in an intermediate point.
Furthermore, the solution presented in DE19526119A requires serrated elements for triggering crushing of the two tubes at their ends, so that it has a relatively high number of components to be produced and assembled.
In addition, during and at the end of impact, the second tube is substantially free to come out, whereas it is necessary for it to remain stationary with respect to the first tube. In fact, in the case of a head-on collision between two carriages, the anti-climbing plates of their collapsible elements may uncouple from one another on account of one or more rebounds (due in particular to a pre-set failure of the interconnection elements between the bodies of the carriages), but they must not change position so that they can couple up again together and continue to perform their function properly.